1. Field of the Invention
The present invention relates to detecting a failed thyristor, and more particularly to detecting a failed open thyristor in reduced voltage solid-state motor starters.
2. Description of the Related Art
Electric motors often use xe2x80x9cthyristors,xe2x80x9d which are also known as xe2x80x9csilicon controlled rectifiersxe2x80x9d (xe2x80x9cSCRsxe2x80x9d), as part of the motors"" control circuitry. A thyristor can be thought of as a switchable diode with three terminals: a gate, an anode, and a cathode. If a supply voltage that is less than a breakdown voltage is applied across the anode and cathode of the thyristor, and no xe2x80x9ctriggerxe2x80x9d current or voltage (trigger signal) is applied to the gate, the thyristor is xe2x80x9coff,xe2x80x9d i.e., no current flows from the anode to the cathode. If a trigger signal is applied to the gate, the voltage across the anode and cathode of the thyristor drops to a very low value in comparison to the supply voltage, and the thyristor turns xe2x80x9con,xe2x80x9d i.e. current flows through the thyristor from the anode to the cathode. Once on, the thyristor can remain on, provided the current through the thyristor remains above a holding current, regardless of the trigger signal at the gate. For the thyristor to turn off, the anode to cathode current must be reduced to a level below the holding current value for the device.
As is well known in the art, solid state starters, or controllers, control electric current flow from a power supply to the motor while the motor is starting. These starters have thyristor switches that gradually increase the current delivered to the motor. Using the thyristor switches, the starter regulates the time period that the thyristors conduct electricity and pass current. In other words, the starter controls when the current from the power supply is delivered to the motor. By controlling the current supplied to the motor during startup, the motor is gently brought up to full operating speed.
When an electric motor is started without such a starter, current drawn by the motor can be excessive, typically six times the steady state current, i.e., the current once it reaches full operating speed. This large current inrush can cause a voltage drop in the power distribution system, causing lights to dim and flicker and disturbing nearby equipment. In addition, the motor torque may rise quickly and oscillate, which can adversely affect the mechanical components of the motor or anything coupled to it.
Failure of a thyristor in the starter may also result in poor motor functioning. Thyristor failures generally result in unbalanced power supply conditions, which may lead to large torque oscillations that can damage mechanical couplings and gears driven by the motor. Some present day thyristor failure detectors use an electronic circuit intended to detect an open thyristor fault, i.e., when the thyristor fails to conduct when it is intended. These thyristor failure detectors indirectly measure three currents through three supply lines by measuring three voltages generated by current transformers in the supply lines. The three voltages are rectified and summed. This summed signal ideally falls within a certain range, which characterizes the correct operation of the system. If the summed signal falls out of this predetermined range, and such a situation persists for a predetermined period of time, the detector signals a fault. This detector circuit assumes that, in the case of a thyristor failure, the motor current waveform is distorted in a way that causes an excessive ripple in the summed signal. This assumption, however, has two problems which may cause malfunctioning in the detector.
First, the motor current waveform may be distorted for other reasons than a faulty thyristor. For example, the motor may operate in magnetic saturation. In such a case, the ripple affecting the summed signal may cause the fault detector to falsely detect a failure condition. Second, if a thyristor fails open, the distorted summed signal may last for a time period that is much shorter than the predetermined time, and the failure goes undetected. Shortening the predetermined time would only increase the sensitivity of the detection circuit and may result in false detections.
Therefore, there is a need to detect quickly a failed-open thyristor during operation of the motor without creating false detections.
A method consistent with this invention detects if a thyristor failed open in a solid-state controller, or starter, for delivering power to a load by an input. The method comprises measuring an instantaneous power delivered to the load during a cycle of the input, determining a peak power delivered to the load over the cycle of the input, calculating an average power delivered to the load over the cycle of the input, and determining if the thyristor failed open by comparing the magnitudes of the peak power and the average power.
An apparatus consistent with this invention detects if a thyristor failed open in a solid-state controller for delivering power to a load by an input. The apparatus comprises a power meter for measuring an instantaneous power delivered to the load during a cycle of the input; a memory containing a program configured to determine a peak power delivered to the load over the cycle of the input, calculate an average power delivered to the load over the cycle of the input, and determine if the thyristor failed open by comparing the magnitudes of the peak power and the average power; and a processor for running the program.
The summary and the following detailed description should not restrict the scope of the claimed invention. Both provide examples and explanations to enable others to practice the invention. The accompanying drawings, which form part of the detailed description, show one embodiment of the invention and, together with the description, explain the principles of the invention.